Resources

Projects

In this e-mail, David Woodhouse described issues with implementing support for KERNEL XIP in flash. The requirements here are a bit different from supporting KERNEL XIP in ROM, since the flash may be unreadable during certain flash operations. Therefore, portions of the kernel must be copied to RAM, and certain kernel operations must be disallowed when the flash is unavailable.

Specifications

TODO: list or link to CELF specifications related to this technique

Patches

Kernel 2.6.10 now includes XIP support:

ARM PATCH 2154/2: XIP kernel for ARM

Patch from Nicolas Pitre

This patch allows for the kernel to be configured for XIP. A lot of people are using semi hacked up XIP patches already so it is a good idea to have a generic and clean implementation supporting all ARM targets. The patch isn't too intrusive.

It involves:

modifying the kernel entry code to map separate .text and .data sections in the initial page table, as well as relocating .data to ram when needed

modifying the linker script to account for the different VMA and LMA for .data, as well as making sure that .init.data gets relocated to ram

adding the final kernel mapping with a new MT_ROM mem type

distinguishing between XIP and non-XIP for bootmem and memory resource declaration

and adding proper target handling to Makefiles.

While at it, this also cleans up the kernel boot code a bit so the kernel can now be compiled for any address in ram, removing the need for a relation between kernel address and start of ram. Also throws in some more comments.

And finally the _text, _etext, _end and similar variables are now declared extern void instead of extern char, or even extern int. That allows for operations on their address directly without any cast, and trying to reference them by mistake would yield an error which is a good thing.

Tested both configurations: XIP and non XIP, the later producing a kernel for execution from ram just as before.

Case 3 - comparing NOR XIP with OneNAND quick-copy to RAM

As I've mentioned in AG meeting, we've done some boot time measurements on OMAP 5912 target platform (OSK5912 from Spectrum Digital). We've done this experiment in order to identify the timing gap between NOR XIP and NAND shadowing. Here is the result (the number represents time in microseconds).

The column noted as "XIP tuning" means that we changed the NOR I/F setting of OMAP (EMIFS) so that the synchronous read is used instead of (default) asynchronous read.

In case of OneNAND, only 1Kbytes of initial part of OneNAND can be used as XIP region and we used 1Kbytes IPL for loading u-boot. Shadowing means that kernel copy (to RAM) is used.

The reason why the kernel initialization time are broken into 2 phases is that we used timer register for measurement and the timer is initialized during kernel booting. You can just add the values for 2 phases to get the total kernel booting time.

Implementation Notes (from the field)

Notes on configuring Linux for XIP (for PPC)

Using XIP with U-Boot on Arm

Wolfgang Denks, the primary author of the UBoot bootloader, wrote the following:

>> Yes. But... _Does_ mkimage -x put header on the front of it?
Yes, it does.
>>> > * You program the resulting image at 0x10004000.
>>> >
>>> > What is programmed at 0x10004000 ? The xipImage code or the uboot header?
>
>>
>> The u-boot headers, yes. Thats wrong. But how to use mkimage -x then?
>> Is the header-caused offset known?
Yes. The U-Boot header is 64 bytes.
U-Boot expects (and verifies) that the entry point is equal to the load address plus the size of the U-Boot header.

Lots more details are in the thread (split across months in the archives):

How to determine offsets for sections

Dick Johnson talks about how to set the physical address for ELF sections by
editing the kernel link files.

On Fri, 21 Oct 2005, Sreeni wrote:
>> Hi,<br>
>><br>
>> I have a montavista XIP kernel running on ARM and my kernel will be in<br>
>> the flash. Since its XIP, I know that the ".text" portion of the<br>
>> kernel will be executed from flash but that ".data" needs to be placed<br>
>> in SDRAM. Now my question is - based on what offset this data will be<br>
>> placed?<br>
>><br>
>> My SDRAM physicall address starts at 3000_0000 and flash starts at<br>
>> 0100_0000. when i allocated a global variable in the kernel module and<br>
>> when i try to check its actually physical address using virt_to_phys,<br>
>> its giving me the address in the range of 0100_0000 ~ 0600_0000 which<br>
>> is my flash (the PAGE_OFFSET doesn't work in case of XIP).<br>
>><br>
>> Can you please help in knowing the physical address of my .data<br>
>> portion in this situation.<br>
>><br>
>> Thanks<br>
>> Shree<br>
</code><br>
I don't know about the ARM in particular, but if you look in ../arch/arm/boot/compressed/vmlinux.lds.in, you will see that this linker-file simply allocates the start addresses of each section as the next available address. The same is true of ../arch/arm/boot/bootp.lds. If you expect to have code the data elements and stack accessed at a specific physical offset, you modify the linker files().
Note that "." means "right here", just like '$' in many assemblers. You can specify a physical offset simply as:
ENTRY(_start)
SECTIONS
{
. == 0x01000000 <==== like this for code
.text : {
...
... }
.rodata : { }
. == 0x30000000 <==== like this data
.data : { }
.bss : { }
}
In the above, we have put .rodata (initialized ASCII stuff) right after the code in the .text section. You may need to extract this from the binary blob to put into your NVRAM.
Also, any initialzed data needs to be relocated to your writable SDRAM and the .bss stuff needs to be zeroed. This is non-trivial. You may want to create a ".reloc" section which contains your initialized data, put it in your flash, and relocate it at startup.
...
Cheers,
Dick Johnson